1. Field of the Invention
The present invention generally concerns the molding of articles from synthetic material subject to shrinkage of setting and is more particularly directed to the molding of optical components, such as ophthalmic lenses, for example, from a synthetic material of this kind.
2. Description of the Prior Art
As is shown, ophthalmic lenses may be molded from polycarbonate, for example, the shrinkage of which on setting is of the order of 1% by volume; lenses may also be molded from 2-allyl carbonate of glycol diethylene, better known under the trade designation "CR-39", the volume shrinkage of which on setting is of the order of 14 to 15%, much higher than the previous figure.
Generally speaking, the molding device usually utilized to mold ophthalmic lenses comprises two spaced mold parts in face-to-face relationship.
The mold parts utilized may be held spaced from one another by a spacing annular seal at their perimeter, for example, and each may consist of a dished member of tempered glass, this being a material capable of providing an optically perfect surface finish essential to achieving the required finish on the ophthalmic lens to be molded and which, while being rigid, offers sufficient elasticity to permit the mold parts to "follow" the shrinkage of the synthetic material contained between them as it sets.
However, in the case of molding ophthalmic lenses, the thickness of the synthetic material between the mold parts utilized is not the same at all points: it is greater at the perimeter than at the center in the case of a divergent ophthalmic lens and it is greater at the center than at the perimeter in the case of a convergent ophthalmic lens.
The result is differential linear shrinkage in the molded ophthalmic lens, parallel to its axis, that is to say linear shrinkage which is not the same at all points since it is proportional to the thickness.
Because of such differential shrinkage during molding, ophthalmic lenses of synthetic material are usually subject to stresses which may render them fragile and which can be prejudicial to certain treatments applied to them, such as a coloring treatment, for example.
French Pat. No. 1 278 497 proposes a molding device in which each of the mold part comprises, in combination, a rigid supporting base and a rigid molding die of an elastically deformable material, said molding die being separated from said base so as to be able to follow the molded material as it shrinks.
An arrangement of this kind complicates the manipulation of the mold parts, as they are no longer of unitary construction, and in practice calls for the use of a guide at the perimeter of the molding dies. Nor is it able to ensure an even distribution of the pressure at all points on the molding dies.
This may still result in significant tensile stresses in the ophthalmic lenses concerned.
A general object of the present invention is to provide a molding device with which this disadvantage may be overcome and which can offer further advantages.